Fires in areas that are partially or totally confined are extremely difficult to extinguish due to a number of factors including, for example, heat buildup, the availability of fuel and the presence of toxic gases, all of which make delivery of fire suppressant material and extinguishing of the fire difficult. Confined areas include, but are not limited to, for example, large furnaces, autoclaves, storage tanks and subsurface structures such as subway and highway tunnels, underground mines and landfills. In coal mine fires, for example, the abundance of fuel in a confined, poorly accessible area practically guarantees that the fire will burn for extremely long periods of time. Historically, many coal mines are abandoned with the onset of a fire because of the great difficulty in extinguishing the fire. One such example is the Jonesville coal mine fire, which started more than 30 years ago and is still burning.
Efforts have been made to prevent fires within such confined areas by introducing an inert gas to displace oxygen within the atmosphere of the confined area to eliminate, or at least substantially diminish, a required fuel source for the fire. The use of an inert gas to prevent combustion is well known. Similarly, it is well known to use a pressure swing adsorption (PSA) system for providing an enriched gas to inert an atmosphere of interest.
As is generally known, a gas mixture may be separated using PSA technology by passing the mixture at an elevated pressure through an adsorbent that is selected in accordance with its capacity to adsorb one or more of the components of the mixture. This selectivity is governed by pore size distribution in the adsorbent and the total pore volume. Accordingly, gas molecules with a kinetic diameter less than or equal to the pore size of the adsorbent are retained, or adsorbed, on the adsorbent while gas molecules of larger diameters pass through the adsorbent. The adsorbent, in effect, sieves the gas according to its molecular size. For example, carbon molecular sieves may be used for the production of enriched nitrogen from air as the carbon molecular sieves have a pore structure with a diameter comparable to the kinetic diameter of oxygen. Accordingly, oxygen is adsorbed by the carbon molecular sieve while nitrogen passes through the sieve.
While PSA systems are known, there are perceived problems with conventional PSA systems and molecular sieve technology such as, for example, low yield of the enriched product gas, requirements for large volumes of sieve materials to provide effective capacity within the absorbent, inefficient regeneration methods for refreshing the absorbent, and the requirement for costly vacuum and air receiver systems to cycle air within the system. Accordingly, the inventors have discovered that use of a PSA system and, in particular, a PSA nitrogen generator for providing an enriched nitrogen gas to inert a large volume atmosphere such as is defined by, for example, a mine, can be optimized to maximize the efficient of the system and achieve the desired effect, while also reducing operating cost. Moreover, the inventors have discovered that an improved PSA nitrogen generator is an effective tool for preventing fires within confined areas such as, for example, mines